Surface cleaning is a fundamental requirement for many industrial processes. It is also important for decontamination of objects. Traditionally, surface cleaning has been accomplished using solvent-based methods, technologies which have been available for more than 100 years. Increasing concerns about ground water and air pollution, greenhouse gases, and related health and safety issues have severely restricted the use of common volatile organic solvents, and even many of the recently-adapted, less hazardous chemical substitutes.
Plasmas have been used extensively in a wide variety of industrial and high technology applications, from semiconductor fabrication to coatings of reflective films for window panels and compact disks. Plasmas ranging in pressure from high vacuum (&lt;0.1 mTorr) to several Torr are most common, and have been used for film deposition, reactive ion etching, sputtering and other forms of surface modification. The primary advantage of plasma cleaning is that it is an "all-dry" process, generates minimal effluent, does not require hazardous pressures, and is applicable to a wide variety of vacuum-compatible materials, including silicon, metals, glass, and ceramics.
Plasma cleaning, typically involving O.sub.2 plasmas, is often used as a means of in-situ surface cleaning and is especially effective against hydrocarbon and other organic surface contaminants. Studies of O.sub.2 plasmas used for surface cleaning have shown that atomic oxygen and O.sub.2.sup.+ is especially reactive to organic contaminants. However, because of the short lifetime of these reactants and their line-of-sight reactivity on the surface, these highly activated reactants are not especially well-suited for surface cleaning of irregular surfaces, especially unpolished wall surfaces or other roughened surfaces. Cleaning of surfaces with nicks, scratches, or anodized surfaces requires long-lived reactive species that can diffuse into minute scratches and openings without loss of chemical activity.
Use of a plasma at or above ambient atmospheric pressure, by contrast, does not require the article to be evacuated, thereby significantly reducing processing cost, and removes the requirement that the article to be cleaned must be capable of surviving under reduced pressure. To date, the problem with atmospheric pressure discharges has been the rapid recombination of atomic oxygen and O.sub.2.sup.+ at this pressure. However, metastable oxygen (.sup.1 .DELTA..sub.g O.sub.2), formed in a plasma has a lifetime ranging from 0.1 sec (at atmospheric pressure) to 45 min. (at zero pressure), and also has 1 eV of internal energy to promote its chemical reactivity. Metastable oxygen production in plasmas is increased at higher pressures due to the three-body reaction: 2O+O.sub.2 .fwdarw.O.sub.2 (.sup.1 .DELTA..sub.g)+O.sub.2. Atomic oxygen is locally produced in the plasma from dissociation of ground-state molecular oxygen. Metastable oxygen also produced directly by the reaction O.sub.2 +e.fwdarw.O.sub.2 (.sup.1 .DELTA..sub.g), where the electron temperature has been optimized around 1 eV. Use of metastables including metastable O.sub.2 for cleaning surfaces is a new concept, and permits plasma processing of both vacuum compatible and incompatible materials at reduced cost and complexity.
Atmospheric pressure plasma torches and flames rely on high-power dc or rf discharges and thermal ionization, respectively, operate at high temperatures, and produce substantial ionization. These plasmas destroy most surfaces they are applied to since the plasmas operate at extremely high temperatures and produce significant concentrations of ions.
Electric-discharge "plasma cells" are dielectric barrier discharge cells often referred to as ozonizer cells, since they are widely used in the industrial generation of ozone. See, e.g., T. C. Manley, "The Electric Characteristics of the Ozonator Discharge," Trans. Electrochem. Soc. 84, 83 (1943). Multiple, self-terminating microdischarges occur throughout the discharge volume as a result of the application of an alternating high-voltage waveform to one of the two electrodes. The feed gas typically contains oxygen and/or water vapor; highly reactive O and OH radicals being produced therefrom in the microdischarges. Significant concentrations of ions are also generated by the microdischarges.
In "Atmospheric Pressure Plasma Pen Driven by a Surfatron," by J. Heidenreich et al., IBM Technical Disclosure Bulletin 31, 234 (February 1989), a submillimeter dimension atmospheric plasma jet operating on argon, nitrogen and/or oxygen is described. This device was found to etch polymer-coated substrates at approximately 25 .mu.m/min. under conditions of 200 W microwave power with about 21/min. flow of argon, and permits the selective removal of polymer debris without the requirement of a vacuum system to create the plasma. However, the apparatus, which relies on surface wave generation, requires microwave frequencies, thereby presenting a health safety problem. Moreover, the plasma is generated over a 1 cm diameter area, is quite hot, and contains a significant concentration of ions.
In U.S. Pat. No. 5,414,324 for "One Atmosphere, Uniform Glow Discharge Plasma," which issued to John R. Roth et al. on May 9, 1995, a one-atmosphere, steady-state glow discharge plasma within the volume between a pair of insulated, equally spaced planar metal electrodes energized with an rms potential of 1-5 kV at 1-100 kHz is described. Roth et al. states that glow discharge plasmas are produced by free electrons which are energized by imposed direct current or rf electric fields, and then collide with neutral molecules, and that these neutral molecule collisions transfer energy to the molecules and form a variety of active species which may include photons, metastables, atomic species, free radicals, molecular fragments, monomers electrons, and ions. Surrounding the plate assembly is an environmental isolation enclosure in which a low feed gas flow is maintained in order to equal the leakage rate of the enclosure. In fact, a no flow condition is taught for normal operation of the apparatus. Materials may be processed by passing them through the plasma between the electrodes, where they are exposed to all plasma constituents including ions. See, e.g., U.S. Pat. No. 5,403,453 for "Method And Apparatus For Glow Discharge Plasma Treatment Of Polymer Materials At Atmospheric Pressure," which issued to John R. Roth et al. on Apr. 4, 1995, and U.S. Pat. No. 5,456,972 for "Method And Apparatus For Glow Discharge Plasma Treatment Of Polymer Materials At Atmospheric Pressure," which issued to John R. Roth on Oct. 10, 1995.
Accordingly, it is an object of the present invention to generate a low-temperature, atmospheric pressure plasma having substantial output of metastable and radical reactive species over a significant area.
Another object of the present invention is to generate a low-temperature atmospheric pressure plasma having substantial output of nonionic reactive species over a significant area.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.